For posterior spinal column correction, there are fundamentally two types of fixation or anchoring devices, pedicle screws and spinal column hooks. As their name implies, pedicle screws are inserted into the pedicle of the spinal column. Pedicle hooks can be attached either to the pedicle or to the lamina of the spinal body. They are placed primarily in the thoracic area of the spine. For the most part, screws, which are inserted into the spinal body, are used in anterior spinal column correction.
Spinal column fixation devices of this type in the form of a pedicle screw are described, for example, in WO 92/03100.
The problem with all previously known spinal column systems for treatment of spinal column deformities, is that their application is complex. This problem shows up primarily when the individual spinal column fixation devices must be linked with a longitudinal support piece. In contrast to treatment of spinal column fractures, in treating deformities, several of the anchoring devices must be attached to the spinal column. Since the spinal column is deformed, only in very rare cases is it possible to place these anchoring devices in a line. From this comes the problem of connection to a longitudinal support piece which normally consists of a round rod.
In the so-called derotation technique, the longitudinal support piece is pre-bent, and after attachment to the anchoring devices, turned 90 degrees. This results in transformation of a lateral bending of the spinal column into a bending in the sagittal plane. In spite of the initial enthusiasm for this technique, it has been determined that the long term result is only a slight correction.
A correction of the spinal column which is optimal according to current state of the art can be achieved with the so-called frame technique. In this technique, the portion of the spinal column that is to be corrected is bridged by two longitudinal support pieces. Both longitudinal support pieces are attached to the spinal body adjacent to the deformation. In accord with the type of correction desired, the two longitudinal support pieces can be pre-bent. Into the spinal bodies which are bridged by the two longitudinal support pieces, anchoring devices (pedicle screws or pedicle hooks) are attached. With the help of these devices, the individual spinal bodies can now be pulled toward the longitudinal support pieces and attached.
In spite of its advantages relative to all previously known methods, this technique can very seldom be used, because the anchoring devices do not possess sufficient adaptability. In addition to the axial loading, which is desired, they cause unnecessary bending loads, since they must be rigidly linked to the longitudinal support piece. These additional loads are so great that at older defective locations in the spinal column, they can lead to a tearing out of the anchoring devices. This tearing-out problem occurs very frequently, since attachment of the anchoring device to the longitudinal rod can be done only in one position. That is, if the correction of the spinal column by its ossification is not 100% feasible, then this partially completed correction cannot be kept in this position. For this reason, often too much force is applied to achieve 100%, and indeed not because of the correction, but rather because of the required attachment of the anchoring devices to the longitudinal support piece. The only alternative is the intermediate insertion of a third longitudinal support piece, which must be attached to an existing longitudinal support piece via a connecting piece. This quantity of implants, however, often exceeds the biological tolerance limit and impairs the functioning of the spinal column musculature.
Presently known individual attachment devices (for example, in accord with WO 92/03100) have an improved adaptation, but are at the tolerance limit especially in young patients, because of their size. A large size proves to be negative above all in complex corrections, since almost every spinal column which is equipped with such an implant, and the spinal column musculature, suffers from it.